Prior to the past several decades, scientists generally believed that it was impossible to restore hearing to the profoundly deaf. However, scientists have had increasing success in restoring normal hearing to the deaf through electrical stimulation of the auditory nerve. The initial attempts to restore hearing were not very successful, as patients were unable to understand speech. However, as scientists developed different techniques for delivering electrical stimuli to the auditory nerve, the auditory sensations elicited by electrical stimulation gradually came closer to sounding more like normal speech.
The electrical stimulation is implemented through a prosthetic device, known as a cochlear implant, which is implanted in the inner ear to restore partial hearing to profoundly deaf patients. Such cochlear implants generally employ an electrode array that is inserted into the cochlear duct. One or more electrodes of the array selectively stimulate different auditory nerves at different places in the cochlea based on the pitch of a received sound signal.
Within the cochlea, there are two main cues that convey “pitch” (frequency) information to the patient. There are (1) the place or location of stimulation along the length of a cochlear duct and (2) the temporal structure of the stimulating waveform. Sound frequencies are mapped to a “place” in the cochlea, generally from low to high sound frequencies mapped from the apical to basilar direction. The electrode array is fitted to the patient to arrive at a mapping scheme such that electrodes near the base of the cochlea are stimulated with high frequency signals, while electrodes near the apex are stimulated with low frequency signals. The stimulation signals provided to the electrodes model the received acoustic signal associated with a particular frequency band. However, the stimulation signals do not necessarily accurately represent the signals generated by nerves.
FIG. 1 presents a cochlear stimulation system 10 that includes a sound processor portion 12 and a cochlear stimulation portion 20. The sound processor portion 12 includes a microphone 14 and a sound processor 18. The microphone 14 can be connected directly to the sound processor 18. Alternatively, the microphone 14 can be coupled to the sound processor 18 through an appropriate communication link 16. The cochlear stimulation portion 20 includes an implantable cochlear stimulator 22 and an electrode array 24. The electrode array 24 is adapted to be inserted within the cochlea of a patient. The electrode array 24 includes a plurality of electrodes (not shown) that are distributed along the length of the array and are selectively connected to the implantable cochlear stimulator 22.
The electrode array 24 may be substantially as shown and described in U.S. Pat. No. 4,819,647 or U.S. Pat. No. 6,129,753, both patents incorporated herein by reference. Electronic circuitry within the implantable cochlear stimulator 22 allows a specified stimulation current to be applied to selected pairs or groups of the electrodes (not shown) included within the electrode array 24 in accordance with a specified stimulation pattern defined by the sound processor 18.
The sound processor 18 and the implantable cochlear stimulator 22 are electronically coupled through a suitable communication link 26. In an implementation, the microphone 14 and the sound processor 18 comprise an external portion of the cochlear stimulation system 10, and the implantable cochlear stimulator 22 and the electrode array 24 comprise an internal, or implanted, portion of the cochlear stimulation system 10. Thus, the communication link 26 is a transcutaneous (through the skin) link that allows power and control signals to be sent from the sound processor 18 to the implantable cochlear stimulator 22.
In another implementation, the implantable cochlear stimulator 22 can send information, such as data and status signals, to the sound processor 18 over the communication link 26. In order to facilitate bidirectional communication between the sound processor 18 and the implantable cochlear stimulator 22, the communication link 26 can include more than one channel. Additionally, interference can be reduced by transmitting information on a first channel using an amplitude-modulated carrier and transmitting information on a second channel using a frequency-modulated carrier.
In an implementation in which the implantable cochlear stimulator 22 and the electrode array 24 are implanted within the patient, and the microphone 14 and the sound processor 18 are carried externally (not implanted) by the patient, the communication link 26 can be realized though use of an antenna coil in the implantable cochlear stimulator 22 and an external antenna coil coupled to the sound processor 18. The external antenna coil can be positioned so that it is aligned with the implantable cochlear stimulator 22, allowing the coils to be inductively coupled to each other and thereby permitting power and information, e.g., a stimulation signal, to be transmitted from the sound processor 18 to the implantable cochlear stimulator 22. In another implementation, the sound processor 18 and the implantable cochlear stimulator 22 can both be implanted within the patient, and the communication link 26 can be a direct-wired connection or other suitable link as shown in U.S. Pat. No. 6,308,101, incorporated herein by reference.
In the cochlear stimulation system 10, the microphone 14 senses acoustic signals and converts the sensed acoustic signals to corresponding electrical signals. The electrical signals are sent to the sound processor 18 over an appropriate communication link 16, such as a circuit or bus. The sound processor 18 processes the electrical signals in accordance with a sound processing strategy and generates control signals used to control the implantable cochlear stimulator 22. Such control signals can specify or define the polarity, magnitude, location (which electrode pair or group is intended to receive the stimulation current), and timing (when the stimulation current is to be applied to the intended electrode pair or group) of the stimulation signal, such as a stimulation current, that is generated by the implantable cochlear stimulator 22.
It is common in the cochlear stimulator art to condition the magnitude and polarity of the stimulation current applied to the implanted electrodes of the electrode array 24 in accordance with a specified sound processing strategy. A sound processing strategy involves defining a pattern of stimulation waveforms that are applied as controlled electrical currents to the electrodes of an electrode array 24 implanted in a patient. Stimulation strategies can be implemented by modulating the amplitude of the stimulation signal or by modulating the frequency of the stimulation signal.